In an MR imaging (MRI) system or MR scanner, an examination object, usually a patient, is exposed within the examination space of the MRI system to a uniform main magnetic field (B0 field) so that the magnetic moments of the nuclei within the examination object tend to rotate around the axis of the applied B0 field (Larmor precession) resulting in a certain net magnetization of all nuclei parallel to the B0 field. The rate of precession is called Larmor frequency which is dependent on the specific physical characteristics of the involved nuclei and the strength of the applied B0 field.
By transmitting an RF excitation pulse (B1 field) which is orthogonal to the B0 field, generated by means of an RF transmit antenna or coil, and matching the Larmor frequency of the nuclei of interest, the spins of the nuclei are excited and brought into phase, and a deflection of their net magnetization from the direction of the B0 field is obtained, so that a transversal component in relation to the longitudinal component of the net magnetization is generated.
After termination of the RF excitation pulse, the relaxation processes of the longitudinal and transversal components of the net magnetization begin, until the net magnetization has returned to its equilibrium state. MR (relaxation) signals which are emitted during the relaxation processes, are detected by means of an RF/MR receive antenna or coil.
The received MR relaxation signals which are time-based amplitude signals, are Fourier transformed to frequency-based MR spectrum signals and processed for generating an MR image of the nuclei of interest within an examination object. In order to obtain a spatial selection of a slice or volume of interest within the examination object and a spatial encoding of the received MR relaxation signals emanating from a slice or volume of interest, gradient magnetic fields are superimposed on the B0 field, having the same direction as the B0 field, but having gradients in the orthogonal x-, y- and z-directions.
The above RF (transmit and/or receive) antennas can be provided both in the form of so-called body coils (also called whole body coils) which are fixedly mounted within an examination space of an MRI system for imaging a whole examination object, and as so-called surface or local coils which are arranged directly on or around a local zone or area to be examined and which are constructed e.g. in the form of flexible pads or sleeves or cages like head coils.
Further, such RF transmit and/or receive antennas can be realized on the one hand in the form of an RF antenna array or array coil, which comprises a number of coil elements which are individually selected for being driven by an RF current source in order to generate (and/or receive) their own local magnetic field such that a desired overall magnetic field distribution is generated within the examination space by all coil elements. However, this requires that the coil elements are decoupled from each other, or the mutual couplings (mainly due to magnetic flux) between the elements are compensated. On the other hand, an RF transmit and/or receive antenna can be realized in the form of an RF resonator, especially an RF volume resonator, which comprises a number of conductor elements which are electromagnetically coupled to each other such that by driving the RF resonator at one or two ports by an RF current source, a number of linearly independent resonant current distributions (“resonant modes”) can be excited in the RF resonator for generating magnetic fields at certain resonance frequencies in a volume of interest (usually an examination space).
U.S. Pat. No. 7,285,957 discloses a multi-port RF birdcage coil assembly which comprises a coil structure having a number of coil elements extending between an inferior and a superior end-ring, wherein capacitors being connected with the end-rings, and a drive network being provided with multiple drive ports at the coil structure which are configured to drive the coil structure at more than two points on one of the end-rings with phase-shifted voltages at the same time such that an asymmetrical loading of the coil by a patient as a result of patient asymmetry is reduced and a substantially circular polarization of the field inside the coil structure is maintained. For eliminating standing waves on voltage cables leading to a power source, a balun network is provided, wherein each drive port is connected to a dedicated balun. Further, each balun is fed by a splitter network for receiving, splitting and phase-shifting a voltage input from the power source.